The present invention relates to oxidation of organic contaminants in aqueous mediums using corona induced reactions. More particularly, the present invention relates to the use of a source or means other than oxygen, such as iron, in a corona reactor to facilitate the production of hydroxyl radicals from hydrogen peroxide (H2O2) generated by corona discharges in the aqueous medium to significantly enhance the oxidation of organic contaminants in the aqueous mediums. In addition, the present invention relates to the use of such sources or means in combination with oxygen in corona discharge procedures to even further oxidize organic contaminants in aqueous mediums.
A normal corona discharge is formed when dc or ac high voltage is applied between a non-uniform electrode geometry in a fluid dielectric. An electric corona has a three-dimensional discharge pattern that displays highly localized positive or negative space charge waves. These waves constitute the active region that propagates due to avalanches of electrons present in the high electric field. The electron avalanches are triggered by a photonization mechanism that provides secondary seed electrons. A region of weakly ionized plasma, known as the xe2x80x9cpassive region,xe2x80x9d remains along the track of the wave. This region provides the path for the current flow from the high voltage electrode. This current flow provides energy for the advancement of the corona.
Pulsed streamer corona technology uses high voltage pulses with very short width, approximately 100-1000 ns. This unique characteristic produces a corona that differs markedly from normal continuous discharge (dc corona), ac discharge, and long-pulse (xcx9c1 ms)* corona discharge. In the past, a pulsed streamer corona discharge has been used for treating gas phase pollutants. See, Clements, I. S. et al.: IEEE Transactions Ind. Appl., IA-(23):224 (1987). In the gas phase, pulsed streamer corona was believed to be much more effective at promoting the reactions leading to desulfurization and dentrification than, for example, electron beam processes. See, Clements, I. S. et al.: IEEE Transactions Ind. Appl., IA-(23):224 (1987). This was believed to be attributed to the more efficient production of hydroxyl radicals using pulsed stream corona. In addition, the work on pulsed streamer corona discharge in aqueous solutions with oxygen gas continuously bubbled through the solution has demonstrated the production of large amounts of ozone. See, Clements, I. S. et al.: IEEE Transactions Ind. Appl., IA-(23):224 (1987).
The use of positive pulsed streamer corona for air pollutant treatment has been demonstrated for a number organic and inorganic toxins. U.S. Pat. No. 4,695,358 to Mizuno et al. discloses a method for converting sulfur dioxide and/or nitrogen oxide gases to acid mist and/or particle aerosols in which the gases are passed through a streamer corona discharge zone having electrodes of a wire-cylinder or wire-plate geometry.
Moreover, the use of positive pulsed streamer corona for air pollutant treatment has been demonstrated for a number of organic and inorganic toxins. See Clements, J. S. et al.: in Conf. Record of the IEEE-Indust. Appl. Soc. Ann. Meeting, pp. 1183-1190 (1986); Mizuno A. et al.: Use of an Electron Beam for Particle Charging in: Conference Record of the IEEE-Indus. Appl. Doc. Ann. Meeting. pp 1215-1219. Chicago, Ill. (October 1984); Masuda et al.: Control of NOX by Positive and Negative Pulsed Corona Diarges. Conference Record of the IEEE Indust. App]. Soc. Ann. Meeting, pp. 1173-1182 Denver, Colo. (1986); Moon, et al.: High Efficiency Ozone Generation Using A Helical Strip-Line Electrode and A Fast Rising Pulse Voltage. Conference Record of the IEEE Indust. Appl. Soc. Ann. Meeting. pp 1205-1210. Denver, Colo. (1986); and Chang, J. S., et al.: IEEE Transactions on Plasma Sci., 19(6):1152 (1991). However, in aqueous phase systems, except for the work of Clements, I. S. et al.: IEEE Transactions Ind. Appl., IA-(23):224 (1987), there has been no systematic investigation of pulsed corona in aqueous systems. Other processes that have been used to treat aromatic compounds include the aqueous-phase radiation treatment of benzene and alkyl-substituted benzenes, Nickelsen, N. G. et al.: Environ. Sci. Technol., (26):144 (1991), and anthraquinone dye, Clements, I. S. et al.: IEEE Transactions Ind. Appl., IA-(23):224 (1987), using electron beams and the use of cobalt gamma radiation to treat solutions containing phenol, Micic, O. I., et al.: Radiation Chemical Destruction of Phenol in Aqueous Solution, Radiation for a Clean Environment, International Atomic Energy Agency, Vienna, Austria, IAEA-SN-1194, 233-239 (1975). Clements, I. S. et al.: IEEE Transactions Ind. Appl., IA-(23):224 (1987), have shown that pulsed corona discharges in aqueous solutions with oxygen bubbling through high voltage needle electrodes produces large amounts of ozone that can, in turn, lead to the decolorization of dyes.
The conventional mechanisms by which organic contaminants are degraded are quite varied. For example, molecular ozone can selectively react with contaminants through cycloaddition, electrophilic reaction, and nucleophilic reaction with unsaturated aromatic and aliphatic species. See Langlais, B. et al.: eds., Ozone in Water Treatment, Applications and Engineering, Lewis Publishers, Chelsea, Mich. (1991). In addition, ozone can lead to the formation of hydroxyl radicals. These radicals are highly reactive with a broad range of organic materials, Haag, W. R. et al.: Environ. Sci. Technol., (26):1005 (1992), and they are generally considered crucial for the breakdown of most organic waste contaminants. Hydroxyl radicals are also formed in photocatalytic reactions of hydrogen peroxide, Zepp, R. G. et al.: Environ. Sci. Technol., (26):313 (1992), nitrates, Zepp, R. G. et al.: Environ. Sci. Tech., (21):443-450 (1987), nitrites, Mopper, K. et al.: Science, (250):662 (1990), and semiconductor surfaces, Korman, C. et al.: Environ. Sci. Technol., (22):798-806 (1988); Matthews, R. W.: J. Phys. Chem., (91):3328-3333 (1987); and Davis, A. P. et al.: Wat. Res., (24):53-550 (1990). Photocatalytic processes have also been investigated for the removal of aqueous waste containing metals, such as silver, gold, mercury, cadmium, chromium, copper, nickel and platinum, alone and in combination with organic waste. The effectiveness of most of the above oxidation methods is attributed to the formation of hydroxyl radicals, however, the major problems with sustaining these reactions are radical scavenging by carbonate and other ions in solution and the low selectivity of the reactions.
Another possible way of removing organic contaminants from wastewater is by corona-induced flocculation. This is similar to an alternative treatment strategy proposed for phenol-containing wastes that utilizes hydrogen peroxide and the enzyme peroxidase to polymerize the phenol into colloidal size particles that can be removed by sedimentation or filtration. See, Klibanov, A. N. et al.: Science, (221):259-261 (1983) and Nakamoto, S. et al.: Wat. Res., (26):49-54 (1992). Indeed, radiation processes are commonly used to initiate free radical polymerization for the production of synthetic polymers, and similar radical mechanisms may occur in aqueous corona systems under properly controlled solution conditions.
Ozone is known to also have a strong effect on coagulation or flocculation of organic matter. However, the mechanisms by which ozone facilitate coagulation are not well understood. Indeed there may be several different mechanisms involved that depend upon the characteristics of the waste. See, Langlais, B. et al.: eds., Ozone in Water Treatment, Applications and Engineering, Lewis Publishers, Chelsea, Mich. (1991).
A number of alternative processes have been considered and studied for the degradation of organic contaminants in aqueous solutions. See, Ollis, D. F. et al.: Environ. Sci. Technol., 25(9):1523 (1991). These include oxidation processes such as UV photolysis, direct ozonation, Langlais, B. et al.: eds., Ozone in Water Treatment, Applications and Engineering, Lewis Publishers, Chelsea, Mich. (1991), photo-catalysis, Davis, A. P. et al.: Wat. Res., (24):53-550 (1990) and Okamoto, K-I. et al.: Bull. Chem. Soc. Jpn, (58):2015 (1985), electron beams, Nickelsen, N. G. et al.: Environ. Sci. Technol., (26):144 (1991), and various combinations of these methods. Other processes may include microwave plasma reduction, Barat, R. B. et al.: Environ. Sci. Technol., (23):666 (1989), ultrasonication, Petrier, C. et al.: Environ. Sci. Technol., (26):1639 (1992), and radiation initiated processes such as gamma radiation from cobalt sources, Micic, O. I., et al.: Radiation Chemical Destruction of Phenol in Aqueous Solution, Radiation for a Clean Environment, International Atomic Energy Agency, Vienna, Austria, IAEA-SN-1194, 233-239 (1975) and Hoigne, J.: Radiation for a Clean Environment, International Atomic Energy Agency, Vienna Austria, 219-232 (1975).
Despite advances in the above techniques there still remains a need to develop more efficient, practical, and robust methods to treat the vast quantities of organic waste released to the environment from a wide variety of processes. It is also crucial to develop a fundamental understanding of the chemical reaction pathways involved in the breakdown of organic species in order to develop means of controlling and facilitating these reactions in order to design and operate systems where these reactions can be carried out.
In brief, the present invention alleviates certain of the aforementioned problems and shortcomings of the present state of the art through the discovery of novel methods for degrading organic molecules or contaminants in aqueous mediums. Generally speaking, the methods of the present invention are based upon the realization that corona-induced reactions when supplemented are very effective at breaking down organic contaminants in aqueous mediums. More particularly, the methods of the instant invention are premised upon the realization that sufficient quantities of hydroxyl radicals can be generated from hydrogen peroxide (H2O2) produced by corona discharge procedures, and preferably pulsed streamer corona discharge procedures, through the use of an effective source or means, other than oxygen, in a corona reactor for oxidizing the organic contaminants in the aqueous mediums to end products, such as CO2, H2O and other possible constituents, like HCl. It should be understood that the type of end products generated from the oxidation in accordance with the methods of the present invention will, of course, be dependent upon the types of organic contaminants present in the aqueous mediums.
In accordance with the present invention, it has been surprisingly discovered that the breakdown of the organic contaminants present in the aqueous mediums can be significantly enhanced through the use of corona discharge procedures in combination with such sources or means, other than oxygen, which are capable of facilitating the generation of hydroxyl radicals from the H2O2 produced in the aqueous mediums by the corona discharge processes. Exemplary of such sources or means includes transition metals, such as iron (ferrous or ferric), manganese, copper, cobalt, unanium, rhenium, and other transition metals, elemental iron, photocatalysts, such as titanium dioxide and silicon dioxide,cadmium sulfide, manganese oxide, magnesium oxide, lead oxide and zinc oxide. It should of course be appreciated that while iron is particularly preferred, any source or means, other than oxygen, which is capable of producing hydroxyl radicals is contemplated by the instant invention.
It has also been found in accordance with the methods of the present invention that when oxygen is continuously added to the aqueous mediums in a corona reactor in combination with such sources or means, ozone is formed in-situ by, for example, the pulsed streamer corona, and the breakdown of the organic molecules or contaminants in the aqueous mediums is even further enhanced.
It should therefore be appreciated that the novel and unique methods of the present invention are effective in breaking down organic contaminants, such as aromatics, like phenol, benzene, toluene, ethylbenzene, xylene, anthracene and phenanthracene, halogenated hydrocarbons, like trichloroethylene, tetrachloroethylene, perchloroethylene and other chlorinated and brominated hydrocarbons, nitrogen-containing compounds, such as nitrobenzene and cyanide, sulfur-containing compounds, such as mercaptans and aliphatic compounds, like hydrocarbons, alcohols and carboxylic acids, in aqueous solutions, such as waste waters.